1. Field of the Invention
The present invention relates to a method of exposure and an exposure device for substrates used in manufacturing liquid-crystal display (LCD) elements and semiconductor elements. In particular, the invention concerns a method of exposure and an exposure device for forming a single device pattern by connecting multiple shots on a substrate.
2. Description of Related Art
One of the steps in manufacturing devices such as liquid-crystal displays and semiconductor elements is an exposure step in which the pattern of a reticle or mask (referred to hereinafter as a reticle) is exposed by projection onto a photosensitive substrate (referred to hereinafter simply as a substrate) such as a glass substrate or a wafer coated with resist. One method of projection exposure is known as the "step-and-repeat" method in which, after a pattern formed on a reticle has been exposed onto a specific region of a substrate, the substrate is stepped only a specific distance. The reticle pattern is then exposed again, and so on.
In exposure devices using the step-and-repeat method, to correctly position the reticle and the substrate, a reticle stage displacing the reticle within a plane perpendicular to the optical axis of the projection optical system, a substrate stage displacing the substrate within a plane perpendicular to the projection optical system, and a magnification controller controlling projection magnification of the projection optical system to project the pattern on the mask onto the substrate at a prescribed magnification are provided.
For large surface area LCDs, the LCD device pattern on the substrate is normally formed by a step-and-repeat type exposure device by picture synthesis. In picture synthesis, as shown in FIG. 7, for example, the pattern 10 of a single LCD device is divided into four patterns A, B, D, and D, and exposed. Each of patterns A, B, C, and D corresponds to a single reticle. Normally, the pattern of an LCD device including patterns A, B, C, and D is exposed onto the substrate (glass plate) using several devices at one time. FIG. 8 is an example of the formation of the device patterns 41-46 of six LCDs on a single substrate P. When manufacturing LCDs for TFTs (thin film transistors), 6-7 layers of overlay exposure are conducted during processing. This process may vary depending on device configuration. During pattern exposure, alignment marks ALM1, ALM2, . . . are also exposed. During exposure of the following layer, these alignment marks ALMn (n=1, 2, . . . ) are detected to determine subsequent shot positions.
Even when exposing a pattern such as that shown in FIG. 8 onto substrate P, due to subsequent processing, the substrate P sometimes expands nonlinearly in the manner shown by solid lines in FIG. 9, for example, and becomes deformed (FIG. 9, upper left). When the pattern of a subsequent layer, indicated by the dotted lines in FIG. 9, is overlaid and exposed onto a pattern 51-56 that has been nonlinearly deformed by processing subsequent to exposure, indicated by the solid lines in FIG. 9, a substantial overlay misalignment results in the deformed portion of the device pattern 51.
Methods of high-precision overlaying and exposure of subsequent layers of patterns on the pattern of a deformed substrate P include the enhanced global alignment (EGA) method. In this method, the regularity of a shot arrangement is specifically determined in a statistical manner. This is described, for example, in U.S. Pat. No. 4,780,617. However, in the EGA method, even though it is possible to correct for linear expansion and conduct overlay exposure, significant overlay misalignment occurs in nonlinearly deformed portions.
U.S. Pat. No. 4,833,621 is an example of a method in which the shot position is corrected for each shot, permitting good overlay exposure even when there is nonlinear deformation of the substrate. This method permits exposure while maintaining high overlay precision for each shot, and is useful when configuring one or more devices per shot, as in LSIs. However, this method is not effective for exposure methods in which a single device is configured by multiple shots, such as in picture synthesis in LCDs and the like.
FIG. 10 shows an example of overlay exposure by a method in which the shot position is corrected for each shot (A, B, C, D) as is set forth above. An LCD device pattern is formed by connecting the four patterns A, B, C, and D in a picture synthesis method. In FIG. 10, the device patterns denoted by solid lines represent previous layer patterns that have undergone nonlinear deformation. The patterns denoted by dotted lines represent exposure patterns overlaid thereon. Inspection of a device pattern 61 in the upper left portion of FIG. 10, in which nonlinear deformation has occurred, reveals that each of patterns A, B, C, and D has been overlaid with good precision onto the pattern of the preceding layer. However, picture connection errors at the borders of the synthesized patterns, such as at the border of pattern A and pattern B and the border of pattern A and pattern C, and vertical layer overlay precision errors at picture connections are substantial.
When the picture connection errors at the borders of the synthesized patterns and the overlay errors at picture connection areas are substantial, the performance of the elements formed on either side of the borderline varies greatly. As a result, lines due to variance in contrast at the borderline position appear in the finished LCD.
Further, when the substrate expands due to the effects of other processes, the magnification controller adjusts the magnification of the projection optical system. In conventional exposure devices, the combination precision between adjacent pattern regions is emphasized for each pattern of the first layer. The overlay precision relative to each pattern in the previous layer is emphasized for each pattern of the second layer and so on in the formation of LCD patterns through combination using the picture synthesis method.
In exposure of the second layer and so on, the positions of the alignment marks previously formed on the substrate during the exposure of each pattern are detected, the expansion rate in two dimensions of the substrate is calculated based on the detection results, and the projection magnification and scaling of the substrate stage are altered. The expansion rate (.gamma.X) in the X-axis direction and the expansion rate (.gamma.Y) in the Y-axis direction, which are the directions of movement of the substrate stage, are calculated, and the substrate scale of the substrate stage in the X-axis direction is changed by .gamma.X and in the Y-axis direction by .gamma.Y. The average value, m, of .gamma.X and .gamma.Y (m=(.gamma.X+.gamma.Y)/2) is set as the magnification correction value. Based on the magnification correction value, the magnification controller controls the projection magnification of the projection optical system, thereby ensuring overlay precision.
FIG. 11 shows what happens when pattern exposure from the second pattern on has been conducted by using the prior art method described above. The shot region that was originally the region denoted by dotted line S has expanded into the region S', denoted by the solid line, after exposure of the second layer due to swelling of the substrate caused by processing. In exposure from the second layer on, when the scale in the X-axis direction of the substrate stage is separately changed by .gamma.X, the scale in the Y-axis direction of the substrate stage is separately changed by .gamma.Y, and the projection magnification of the projection optical system is separately changed by (.gamma.X+.gamma.Y)/2, patterns extending beyond the individual shot regions in the X-axis direction are transferred and patterns that are narrower than the individual shot regions only in the Y-axis direction are transferred. In the Lx portion, defining a strip extending in the Y-axis direction, adjacent patterns overlap. In the Ly portion, defining a strip extending in the X-axis direction, a gap is formed between adjacent patterns.
When the overlay precision of each pattern is emphasized in this manner, the connecting portions of the first layer, that is, the pattern overlay in the Lx and Ly strip portions, changes and the characteristics of the element formed by the patterns are altered at this border. Even when these changes in element characteristics are slight, sharp changes on either side of the connecting portion of the first layer cause the changes to appear quite prominently to the human eye as a border line on the liquid-crystal display.